Evaluate the following statements. Do they cause a shift of the aggregate demand curve or a movement along it? Briefly explain.

a. The prices of metals used in batteries, such as lithium and cobalt, surge by about two-thirds and one-third, respectively.

b. Wind and solar firms begin to expand, aided by generous government subsidies.

c. The government raises the value-added tax from 8 percent to 10 percent.

d. The government buys two million vials of COVID-19 vaccines.